1. Field of the Invention
The present invention relates to a high-pressure discharge lamp for emitting light due to a plasma discharge in the presence of a mercury gas atmosphere, and a method of manufacturing such a high-pressure discharge lamp.
2. Description of the Related Art
General high-pressure discharge lamps have a lamp bulb of silica glass with a substantially spherical discharge space defined centrally in its longitudinal direction, and a pair of electrodes of tungsten disposed in the discharge space in confronting relation to each other. The discharge space is filled with mercury, a halogen gas, and an inactive gas. The two electrodes are inserted from respective insertion holes defined in the opposite ends of the lamp bulb. The insertion holes are hermetically sealed by rear portion of the respective electrodes which are covered with respective sheets of molybdenum foil serving as a thermal buffer.
When a trigger voltage is applied between the electrodes, a glow discharge is induced between the electrodes in an inactive gas atmosphere in the discharge space, vaporizing the mercury filled in the discharge space. A plasma discharge is generated in a high-pressure mercury gas atmosphere within the discharge space, emitting highly brilliant light of good color-rendering capability.
When the discharge is caused in the discharge space at a high temperature under a high pressure, the tungsten (W) of the electrodes are vaporized and deposited on the inner surface of the bulb wall which defines the discharge space. The conventional high-pressure discharge lamp is therefore problematic in that the tungsten deposited on the inner surface of the bulb wall blackens the bulb wall, lowering the luminance of the discharge lamp. At present, it is customary to encapsulate a halogen gas into the discharge space to prevent the bulb wall from being blackened. The halogen gas filled in the discharge space produces halogen ions which are combined with the vaporized tungsten and deposits the tungsten on the proximal portions of the electrodes at a relatively low temperature. Such a halogen cycle is repeated to prevent the bulb wall from being blackened.
As described above, the halogen gas is effective to prevent the luminance of the discharge lamp from being lowered due to blackening of the bulb wall. However, if the halogen gas is excessively present in the discharge space, then it tends to erode the electrodes and the molybdenum foil, possibly causing the gas to leak from the lamp bulb and rupturing the lamp bulb. There have been developed various techniques for optimizing the concentration of the halogen gas in the discharge space to simultaneously solve the problem of the reduction of the luminance due to blackening and the problem of gas leakage and lamp bulb rupture (see, for example, Japanese laid-open patent publication No. 11-149899 and Japanese patent No. 2829339).
Today, there are demands for high-pressure discharge lamps of smaller size and higher luminance. To meet these demands, it is necessary to either increase the electric power applied to the high-pressure discharge lamps or increase the amount of mercury filled in the discharge space. If the applied electric power or the filled amount of mercury is increased, then an electric power load (thermal load) on the discharge lamp is increased. The electric power load is called a bulb wall load (L) which is expressed by L=P (the electric power applied to the discharge lamp: W)/S (the inner surface area of the bulb wall which defines the discharge space: mm2). Generally, as the bulb wall load L increases, the discharge lamp exhibits a greater tendency to be quickly deteriorated (blackened, whitened, electrodes consumed, etc.) per unit time. The greatest cause of the deterioration is that the halogen cycle does not function properly. Specifically, the halogen cycle functions properly when halogen gas atoms, whose number of moles is greater than the number of moles of tungsten atoms expelled from the electrodes by the discharge, are present in the discharge space. If the number of moles of halogen gas atoms is smaller than the number of moles of tungsten atoms, then the halogen cycle partly fails to function properly, i.e., tungsten atoms occur which cannot be combined with halogen gas atoms. As a result, those tungsten atoms occur which cannot be combined with halogen gas atoms are deposited on the inner surface of the bulb wall which defines the discharge space, causing blackening of the bulb wall. The blackening of the bulb wall induces a reduction in the transparency of the bulb wall (whitening) and an undue consumption of the electrodes.